U.S. patent number 7,283,400 [Application Number 11/228,389] was granted by the patent office on 2007-10-16 for nonvolatile semiconductor storage device.
This patent grant is currently assigned to Hitachi Device Engineering Co., Ltd., Hitachi Ulsi Systems Co., Ltd., Renesas Technology Corp.. Invention is credited to Michitaro Kanamitsu, Takayuki Kawahara, Naoki Kobayashi, Takashi Kobayashi, Shoji Kubono, Hideaki Kurata, Atsushi Nozoe, Shunichi Saeki, Yoshinori Takase, Keiichi Yoshida.
United States Patent |
7,283,400 |
Kurata , et al. |
October 16, 2007 |
**Please see images for:
( Certificate of Correction ) ** |
Nonvolatile semiconductor storage device
Abstract
A nonvolatile memory device of the present invention performs a
programming operation by accumulating a charge in certain
capacitance which is provided for each programming memory cell and
injecting hot electrons generated when the charge is discharged via
the memory cell into a floating gate. Thus, a variation in a
programming characteristic of the nonvolatile semiconductor memory
device is reduced, thereby realizing high-speed programming
operation.
Inventors: |
Kurata; Hideaki (Kokubunji,
JP), Kobayashi; Naoki (Tokyo, JP), Saeki;
Shunichi (Ome, JP), Kobayashi; Takashi
(Tokorozawa, JP), Kawahara; Takayuki (Higashiyamato,
JP), Takase; Yoshinori (Tokyo, JP),
Yoshida; Keiichi (Musashimurayama, JP), Kanamitsu;
Michitaro (Ome, JP), Kubono; Shoji (Ome,
JP), Nozoe; Atsushi (Hino, JP) |
Assignee: |
Renesas Technology Corp.
(Tokyo, JP)
Hitachi Device Engineering Co., Ltd. (Mobara-Shi,
JP)
Hitachi Ulsi Systems Co., Ltd. (Tokyo, JP)
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Family
ID: |
11736900 |
Appl.
No.: |
11/228,389 |
Filed: |
September 19, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060013032 A1 |
Jan 19, 2006 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10250922 |
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6950347 |
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PCT/JP02/00142 |
Jan 11, 2002 |
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Foreign Application Priority Data
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Jan 22, 2001 [WO] |
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PCT/JP01/00133 |
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Current U.S.
Class: |
365/185.29;
257/E21.679; 257/E21.682; 257/E27.103; 365/185.18; 365/185.28 |
Current CPC
Class: |
G11C
11/5628 (20130101); G11C 16/0425 (20130101); G11C
16/0433 (20130101); G11C 16/0491 (20130101); G11C
16/10 (20130101); G11C 16/12 (20130101); G11C
16/16 (20130101); G11C 16/3409 (20130101); G11C
16/3454 (20130101); G11C 16/3459 (20130101); H01L
27/115 (20130101); H01L 27/11521 (20130101); H01L
27/11568 (20130101); G11C 16/0466 (20130101); G11C
16/3404 (20130101); G11C 16/3436 (20130101) |
Current International
Class: |
G11C
16/04 (20060101) |
Field of
Search: |
;365/185.29,185.18,185.28 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hoang; Huan
Attorney, Agent or Firm: Antonelli, Terry, Stout &
Kraus, LLP.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a Continuation of application Ser. No. 10/250,922, filed
Jul. 8, 2003 now U.S. Pat. No. 6,950,347, the entire disclosure of
which is hereby incorporated by reference and wherein application
Ser. No. 10/250,922 is a national stage application filed under 35
USC .sctn.371 of International Application No. PCT/JP02/00142,
filed 11 Jan. 2002, the contents of Application No. PCT/JP02/00142
are incorporated herein by reference in their entirety.
Claims
What we claim is:
1. A nonvolatile semiconductor memory device comprising: a memory
cell including first and second diffusion layers, a charge-storage
portion between the first and second diffusion layers, a gate
between the charge-storage portion and the first diffusion layer,
and a word line formed above the charge storage portion and
controlled separately from the gate; and a bit line connected to
the memory cell; wherein the charge storage portion is injected
with hot electrons which are generated by passing charges
accumulated in stray capacitance of the bit line through the memory
cell at a portion in a semiconductor substrate under a boundary
between the charge-storage portion and the gate when the memory
cell is programmed or erased.
2. A nonvolatile semiconductor memory device according to claim 1,
wherein the charge storage portion is a floating gate.
3. A nonvolatile semiconductor memory device according to claim 1,
wherein the charge storage portion is a silicon nitride film.
4. A nonvolatile semiconductor memory device according to claim 1,
wherein a portion of the stray capacitance is formed of p-n
junction capacitance of a diffusion layer of the memory cell.
5. A nonvolatile semiconductor memory device according to claim 1,
further comprising: an internal power source circuit to generate a
voltage to be applied to the bit line, wherein, when the memory
cell is programmed or erased, the internal power source circuit is
brought to an inactive state.
6. A nonvolatile semiconductor memory device according to claim 1,
wherein, after the programming or the erase is performed a
plurality of times, an operation of verifying a threshold voltage
of the memory cell is performed.
7. A nonvolatile semiconductor memory device according to claim 6,
wherein the number of repeating the programming or the erase is
incremented every time the threshold voltage verification operation
is performed.
8. A nonvolatile semiconductor memory device comprising: a memory
cell including first and second diffusion layers, a charge-storage
portion between the first and second diffusion layers, a gate
between the charge-storage portion and the first diffusion layer,
and a word line formed above the charge-storage portion and
controlled separately from the gate; and a bit line connected to
the memory cell; means for injecting hot electrons into the
charge-storage portion of the memory cell, which hot electrons are
generated by passing charges accumulated in stray capacitance of
the bit line through the memory cell at a portion in a
semiconductor substrate under a boundary between the charge-storage
portion and the gate, when the memory cell is programmed or
erased.
9. A nonvolatile semiconductor memory device according to claim 8,
wherein the charge-storage portion is a floating gate.
10. A nonvolatile semiconductor memory device according to claim 8,
wherein the charge-storage portion is a silicon nitride film.
11. A nonvolatile semiconductor memory device according to claim 8,
wherein a portion of the stray capacitance is formed of p-n
junction capacitance of a diffusion layer of the memory cell.
12. A nonvolatile semiconductor memory device according to claim 8,
further comprising: an internal power source circuit to generate a
voltage to be applied to the bit line, wherein, when the memory
cell is programmed or erased, the internal power source circuit is
brought to an inactive state.
13. A nonvolatile semiconductor memory device according to claim 8,
wherein, after the programming or the erase is performed a
plurality of times, an operation of verifying a threshold voltage
of the memory cell is performed.
14. A nonvolatile semiconductor memory device according to claim
13, wherein the number of repeating the programming or the erase is
incremented every time the threshold voltage verification operation
is performed.
Description
TECHNICAL FIELD
The present invention relates to a nonvolatile semiconductor memory
device and, particularly, to a nonvolatile semiconductor memory
device such as an electrical programmable and erasable flash
memory.
BACKGROUND ART
Among electrical programmable and erasable nonvolatile
semiconductor memory devices, flash memories are known in the art
as the one capable of bulk erasing. Since the flash memories are
excellent in portability as well as impact resistance and capable
of electrical bulk erasing, they are rapidly growing in demand in
recent years as memory devices to be used for personal digital
assistances such as a mobile personal computer and a digital still
camera. One of the important factors in expanding the market for
the flash memories is a reduction in bit cost by way of a reduction
in memory cell area. For example, as is mentioned in "Applied
Physics", Vol. 65, No. 11, pp. 1114 1124, published by The Japan
Society of Applied Physics on Jan. 10, 1996, various memory cell
technologies which realize the reduction in bit cost have
heretofore been proposed.
DISCLOSURE OF INVENTION
A virtual ground type memory cell using a three-layer polysilicon
gate, which is originated with the present inventors, has been
proposed as another memory cell technology (Japanese Patent
Laid-open No. 11-200242).
The above-mentioned memory cell is shown in FIG. 1, wherein (a) is
a plane view and (b), (c), and (d) are cross-sectional views taken
along lines A A', B B', and C C', respectively.
The memory cell has a source-drain diffusion layer 205 formed in a
well 101 formed on the main surface of a semiconductor substrate
100, a first gate (floating gate) 103b, a second gate (control
gate) 111a, and a third gate 107a. The control gates (second gates)
111a of the memory cells are connected to each other along the row
direction (x direction) to form a word line WL.
The floating gate (first gate) 103b is isolated from the well 101,
the third gate 107a, and the word line (control gates) 111a by a
gate insulator film (first insulator film) 102, an insulator film
(third insulator film) 106a, and an insulator film (second
insulator film) 110a, respectively. In addition, the third gate
107a is isolated from the word line 111a by an insulator film
108a.
The source/drain diffusion layers 205 extend along a direction (y
direction) perpendicular to a direction (x direction) along which
the word line 111a extends, thereby serving as a local source line
and a local data line for connecting sources/drains of the memory
cells aligned along the column direction (y direction). In other
words, this nonvolatile semiconductor memory device is formed of
the so-called contactless array wherein a contact hole is not
formed for each memory cell. A channel is formed along a direction
(x direction) perpendicular to the diffusion layers 105. The
source/drain diffusion layers 205 are arranged in such a manner
that a pair of diffusion layers 205 forming a source/drain is
positioned asymmetrically with a floating gate pattern 103a, so
that one diffusion layer overlaps with both the floating gate and
the third gate.
Two end faces of the third gate 107a respectively face two end
faces of the floating gate 103b with the insulator film 106a being
interposed therebetween, the end faces of the floating gate 103b
being respectively perpendicular to the word line 111a and the
channel.
Further, the third gates 107a are respectively buried in gaps
between the floating gates 103b which are disposed along a
direction (y direction) perpendicular to the word line 111a and the
channel. The floating gates 103b are arranged asymmetrically with
the third gates 107a, and the third gates 107a are arranged
asymmetrically with the floating gates 103b.
With this configuration, even if the third gates 107a are provided
in addition to the floating gates 103a and the control gates 111a,
it is possible to double pitches in the word line WL direction (x
direction) and the local data line direction (y direction) as
compared with those of the minimum feature size. Therefore, it is
possible to reduce the memory cell area to the minimum size of
4F.sup.2 (F: minimum feature size) in a cross point type array.
The above-described memory cell enables not only the reduction in
size but also high-speed programming. A voltage application
condition at the time of programming to a memory cell is shown in
FIG. 2(a), and operation schemes are separately shown in FIGS. 2(b)
and 2(c). Refereeing to FIG. 2(b), a positive voltage of about 5 V,
for example, is applied to a diffusion layer Dn, which is a drain
of a selected memory cell M, at a timing of time t0, then a
positive voltage of about 12 V, for example, is applied to a word
line WLn of the selected memory cell M at a timing of time t1, and,
after that, a voltage which is approximately equal to a threshold
voltage of a MOS transistor formed of a third gate, such as 0.6 V,
is applied to a third gate AGe of the selected memory cell M at a
timing of time t2. A diffusion layer Dn-1 which is a source of the
selected memory cell, a well, and an unselected word line WLn+1 are
maintained at 0 V. With the above operation, a large electric field
is formed both laterally and vertically in a channel below a
boundary between the floating gate and the third gate. Thus, in
spite of the smaller channel current, efficiency in generating and
injecting hot electrons are increased, thereby enabling the
high-speed programming. Consequently, it is possible to achieve
parallel programming of memory cells of more than a kilobyte using
an internal power source having a current supply capacity of about
1 mA. It is possible to perform the above operation if the voltage
application timings of the word line WLn, the diffusion layer Dn,
and the third gate AGe of the selected memory cell are exchanged as
shown in FIG. 2(c).
However, some problems will arise in the above-described memory
cell programming method. Since the voltage which is approximately
equal to the threshold voltage of the MOS transistor formed of the
third gate is applied to the third gates AG for the operation in
the above programming method, a dimensional variation and a
variation in applied voltages of the third gates AG significantly
influence a programming characteristic of the memory cell. Shown in
FIG. 3 is a relationship among a voltage to be applied to the third
gate, a channel current, and a gate current. As shown in FIG. 3,
the gate current Ig changes exponentially near the voltage of about
0.6 V of the third gate AG during the operation. For example, if
there is a variation of .+-.0.1 V in the voltages of the third gate
AG, the variation in the gate currents Ig will be about the order
of 1.3.
Further, since the internal power source for supplying the channel
current operates at the time of programming, the third gate AG
voltage may possibly be changed due to noise from the internal
power source. As described above, since the memory cell programming
characteristic is influenced by the third gate AG voltage to a
considerable degree, even smallest driving noise from the internal
power source can cause a variation in the characteristic.
The programming operation is executed in such a manner that an
application of programming bias and a verification of the threshold
voltage are repeated until each of the threshold voltages of the
memory cells for simultaneous programming reaches a desired value.
Therefore, if the variation in memory cell characteristic occurs,
the numbers of repeating the programming bias application and the
threshold voltage verification are increased to lengthen a
programming period. Accordingly, it is expected that the memory
cell programming period will be increased due to variations in the
dimensions of the third gates AG, the variation in voltages applied
to the third gates AG, and the influence of driving noise from the
internal power source.
Further, in order to realize a multi-level storage which is capable
of storing data of 2 or more bits per memory cell, it is necessary
to reduce a width of threshold voltage distribution for each data;
therefore, the variation in programming characteristic will largely
increase the memory cell programming period.
The present invention provides a nonvolatile semiconductor memory
device which is suitable for size reduction and capable of
absorbing a variation in programming characteristics of high-speed
operation memory cells and realizing a fast programming speed.
The above object is attained by the following means. FIG. 3 shows
that the gate current Ig is greatly influenced by the AG bias;
however, injection efficiency .gamma. is less subject to the AG
bias as compared with the gate current. For example, if there is an
AG variation of .+-.0.1 V near an AG voltage of about 0.6 V during
the operation, a variation in the injection efficiency is in the
order of about 0.3. Accordingly, it is possible to suppress the
variation in programming characteristic to a level similar to that
in the injection efficiency by accumulating charges at certain
capacitance and performing the programming by supplying the charges
accumulated in the certain capacitance to a memory cell. Likewise,
it is possible to suppress the variation in programming
characteristic to a level similar to that in the injection
efficiency by performing the programming by accumulating charges in
certain capacitance via the memory cell.
Some points of the present invention are listed up below.
Programming or erasing is performed by discharging or charging a
charge from or to capacitance via a memory cell and injecting hot
electrons to a charge-injected portion. Thus, the high-speed memory
cell programming operation or high-speed memory cell erase
operation is achieved.
By using stray capacitance of a bit line including pn junction
capacitance of a diffusion layer as the capacitance, the above
high-speed operations are achieved without specially changing the
structure of a nonvolatile semiconductor memory device.
A variation in characteristic due to operation noise caused by an
internal power source can be suppressed by maintaining an internal
power source circuit for generating a voltage to be applied to the
bit line in an inactive state when injecting the charges into the
charge-injected portion.
The high-speed operations are more effectively achieved by
performing the threshold voltage verification operation of the
memory cell after repeating the programming operation or the erase
operation a plurality of times and increasing the number of
repeating the programming operation or the erase operation every
time the threshold voltage verification operation is performed.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a cross-sectional view of a flash memory, illustrating a
principle of the present invention;
FIG. 2 includes diagrams illustrating the programming operation of
the flash memory;
FIG. 3 is a diagram illustrating a problem of the flash memory and
a solution to the problem;
FIG. 4 is a circuit diagram illustrating a first embodiment of the
present invention;
FIG. 5 is a diagram showing timings in the circuit diagram of FIG.
4;
FIG. 6 is a diagram showing a memory array structure for explaining
the first embodiment in detail;
FIG. 7 is a diagram illustrating timings in the programming
operation according to the first embodiment;
FIG. 8 is a diagram illustrating timings in a programming operation
according to a second embodiment;
FIG. 9 is a circuit diagram illustrating the first embodiment of
the present invention;
FIG. 10 is a diagram showing timings in the circuit diagram of FIG.
9;
FIG. 11 is a diagram illustrating timings in a programming
operation according to a third embodiment;
FIG. 12 is a diagram illustrating timings in a programming
operation according to a fourth embodiment;
FIG. 13 is a circuit diagram illustrating a fifth embodiment of the
present invention;
FIG. 14 is a diagram illustrating timings in the circuit of FIG.
13;
FIG. 15 illustrates a circuit configuration for realizing the fifth
embodiment, the circuit configuration being different from that of
FIG. 13;
FIG. 16 is a cross-sectional view of a nonvolatile memory according
to a sixth embodiment;
FIG. 17 includes diagrams showing operation voltage conditions of
the flash memory;
FIG. 18 is a circuit diagram illustrating the sixth embodiment;
FIG. 19 is a diagram showing timings in the circuit diagram of FIG.
18;
FIG. 20 is a circuit configuration for realizing the fifth
embodiment, the circuit configuration being different from that of
FIG. 18;
FIG. 21 is diagram showing a threshold voltage distribution
according to a seventh embodiment;
FIG. 22 is a flowchart illustrating a programming method according
to the seventh embodiment;
FIG. 23 is a diagram showing timings in a programming operation
according to an eighth embodiment;
FIG. 24 is a diagram showing timings in the programming operation
according to the eighth embodiment;
FIG. 25 is a flowchart illustrating the programming method
according to the first to eighth embodiments;
FIG. 26 is a flowchart illustrating a programming method according
to a ninth embodiment of the present invention;
FIG. 27 is a diagram showing a threshold voltage distribution for
illustrating a tenth embodiment of the present invention;
FIG. 28 is a flowchart of a conventional programming method of a
multilevel flash memory;
FIG. 29 is a flowchart illustrating a programming method according
to the tenth embodiment of the present invention;
FIG. 30 is a diagram illustrating timings according to the tenth
embodiment of the present invention;
FIG. 31 is a diagram illustrating timings according to the tenth
embodiment of the present invention;
FIG. 32 is a flowchart illustrating a programming method according
to an eleventh embodiment of the present invention;
FIG. 33 is a circuit diagram illustrating a twelfth embodiment of
the present invention;
FIG. 34 is a flowchart illustrating a programming method according
to the twelfth embodiment of the present invention; and
FIG. 35 is a block diagram showing a system according to a
thirteenth embodiment of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of the present invention will hereinafter be described
in detail with reference to the drawings.
<Embodiment 1>
A first embodiment of the present invention will be described with
reference to FIGS. 4 to 7. FIG. 4 is a circuit diagram which is
directed to one of memory cells of the memory array structure shown
in FIG. 2, and FIG. 5 is a diagram showing a programming operation
scheme of the present embodiment. As shown in FIG. 5, an internal
power source PROG for supplying a channel current is activated to 5
V at a timing of time t0; STS and STD, which are the respective
switching MOS on the source side and the drain side of a selected
memory cell, are brought to an ON-state at a timing of t1; and a
programming voltage of 12 V is biased to a word line WL of the
selected memory cell at a timing of time t2. Then, when a node ND
on the drain side of the memory cell is charged to 5 V, STD, which
is the switching MOS on the drain side, is brought to an OFF-state
at a timing of time t3 to be disconnected from the internal power
source PROG. When a voltage of about 0.6 V is biased to a third
gate AG of the selected memory cell at a timing of time t4, a
charge accumulated in the node ND starts to flow toward the source
side via the memory cell. Here, hot electrons generated in a
channel region of the memory cell are injected into a floating gate
to achieve programming. A voltage of the node ND on the drain side
is decreased with the flow of a channel current, but the
programming is effectively achieved during a period when a
horizontal electric field high enough to generate the hot electrons
is generated.
The present embodiment will be described in more detail with
reference to FIGS. 6 and 7. Shown in FIG. 6 is an example of the
memory array structure of FIG. 1, and shown in FIG. 7 is a
programming operation of the present invention. In FIG. 6, nodes
D00 to D03 and D10 to D13, which are sources and drains of memory
cells, are formed of diffusion layer wirings, and 128 memory cells,
for example, are arranged parallelly on one diffusion layer wiring.
The diffusion layer wiring is connected to one another via bit
lines DL0 and DL1 formed of metal and a selected transistor. In
addition, the selected transistor is controlled by signals sent
from ST00, ST01, ST12, and ST13. Further, the diffusion layer
wiring is connected to one another via a common source line and a
selected transistor, and the selected transistor is controlled by
signals sent from ST02, ST03, ST10, and ST11. Used as the common
source line SS is diffusion layer wiring or diffusion layer wiring
whose wiring resistance is reduced by shunting with metal. Although
two lines of diffusion layer wiring are connected to one metal bit
line via two selected transistors in FIG. 6, N lines of diffusion
layer wiring may be connected to one metal bit line via N selected
transistors. Likewise, although two lines of diffusion layer wiring
are connected to one common source line via two selected
transistors, N lines of diffusion layer wiring may be connected to
one common source line via N selected transistors. The metal bit
lines DL0 and DL1 are connected to control circuits PC0 and PC1
respectively via the switching MOS.
The operation of the present invention will be described using a
timing waveform shown in FIG. 7. Hereinafter, it is assumed that a
memory cell to which the programming is performed is a word line
WL00. Firstly, after a programming command and programming data are
inputted, the common source line SS is activated to about 5 V at a
timing t0. Then, TR is activated to connect the bit lines to the
control circuits at a timing of time t1. Here, the control circuits
output to the bit lines voltages corresponding to the programming
data. For example, a voltage of 0 V is applied to the bit line for
the program-selected memory cell, and a voltage of 1 V is applied
to the bit line for the program-unselected memory cell. After that,
the gate signal ST03 of the selected transistor is brought to a
High state at a timing of time t2 to charge diffusion layer wiring
D02 and D04 to 5 V. Then, the gate signal ST01 of the selected
transistor is brought to a High state at a timing of time t3 to
connect the bit lines DL0 and DL1 to the diffusion layer wirings
D01 and D03, respectively. Here, a voltage of each of the
program-selected D01 and D03 is 0 V, while a voltage of each of the
program-unselected D01 and D03 of program unselected data line is 1
V. After activating the selected word line WL00 to a programming
voltage of 12 V, for example, at a timing of time t4, the gate
signal ST03 of the selected transistor is set to LOW at a timing of
time t5 to disconnect the diffusion layer wiring D02 and D04 from
the common source line. After that, a programming voltage of about
0.6 V, for example, is biased to AG01 which is a selected AG gate.
If a memory cell M01 is the program-selected cell, voltages applied
to D01, D02, the word line WL00, and the AG gate are 0 V, 5 V, 12
V, and 0.6 V, respectively; therefore, electrons are injected into
the floating gate. In this case, since D02 is in a floating state
after being charged to 5 V, a voltage thereof is decreased as the
channel current flows to the memory cell M01, the voltage being
ultimately decreased to 0 V. Therefore, the electron injection into
the floating gate occurs as long as the voltage of D02 is the bias
sufficient for the hot electron generation. On the other hand, if
the memory cell M01 is program-unselected, voltages applied to D01,
D02, the word line WL00, and AG01 are 1 V, 5 V, 12 V, and about 0.6
V, respectively; therefore, no channel current flows thereto and no
electron injection into the floating gate occurs. The capacitance
of one diffusion layer wiring is mainly pn junction capacitance,
and, in the array structure of the present invention, the
capacitance is about 0.3 pF.
Next, AG01 is inactivated at a timing of time t7, and WL00 and SS
are inactivated at a timing of time t8. Then, TR is inactivated at
a timing of time t9 to interrupt the connection between the control
circuits and the bit lines, so that the bit lines and the diffusion
layer wiring are discharged to 0V to complete the operation of
injecting electrons into the floating gate.
After that, an operation for verifying whether or not a threshold
voltage has reached a desired value is performed, and the electron
injection operation is repeated for memory cells in which the
desired threshold voltage has not been attained. The programming is
terminated when the desired threshold voltage is attained in each
of the memory cells to which the programming must be performed.
Thus, since a programming variation due to a variation in AG bias
is reduced, thereby reducing the number of programming
verification, it is possible to shorten time required for
programming.
Further, since it is unnecessary to activate the internal power
source for supplying the programming bias of 5 V during the
programming, it is possible to suppress the variation in
programming characteristic caused by operation noise of the
internal power source and to reduce the power consumption by
maintaining the internal power source at the inactive state.
In the foregoing, the operation of increasing the threshold voltage
is described as the programming; however, it is possible to apply
the threshold voltage increasing operation to an erase
operation.
Also, in the case of the multilevel memory, wherein a memory cell
is capable of having two or more threshold voltages, the effect of
the present method becomes more prominent. In the multilevel
memory, since it is necessary to control with high precision the
threshold voltages for data, if the programming variation is great,
the number of threshold voltage verification is increased, thereby
reducing the programming speed. In the present method, the
programming variation is reduced to suppress the number of
threshold voltage verification, thereby increasing the programming
speed.
In addition, the same effect is achieved if the memory cell is the
one wherein the charge accumulation node of the present embodiment
is formed of a silicon nitride film or a multilayer of a silicon
nitride film and a silicon oxide film in place of a polysilicon
film.
Alternatively, the same effect is achieved if the memory cell is
the one wherein the charge accumulation node is formed of a
plurality of polysilicon particles in the form of dots in place of
the polysilicon film.
Further, in the case of using the silicon nitride film or the
dotted polysilicon particles as the charge-injected portion, the
same effect is achieved if the memory cell is the one wherein the
polysilicon gate having the same function as that of the third gate
is provided on each side of the charge-injected portion via a
silicon oxide film. In this case, the charge-injected portion can
discretely retain charges at two places close to the respective
adjacent polysilicon gates, and thus, the multilevel memory is
realized by the different charge accumulation places.
<Embodiment 2>
A second embodiment of the present invention will be described with
reference to FIGS. 6 and 8. Shown in FIG. 8 is a programming
operation scheme of the present embodiment in an array structure
shown in FIG. 6. The array structure of FIG. 6 is the same as that
described in Embodiment 1, wherein it is assumed that a memory cell
to which programming is performed is a word line WL00.
After a programming command and programming data are inputted, ST02
is activated at a timing of time t0 to connect diffusion layer
wiring D01 and D03 to a common source line, respectively. Then, TR
is brought to a High state at a timing of time t1 to connect
control circuits PC0 and PC1 to metal bit lines DL0 and DL1,
respectively. Here, voltages applied from the control circuits are
such that a programming voltage of 5 V, for example, is biased to
the bit line for a program-selected memory cell and a voltage of 0
V is biased to the bit line for a program-unselected memory cell.
After the program-selected bit line is charged to 5 V, a gate
signal ST00 of a selected transistor is brought to an ON-state at a
timing of time t2 to connect the bit line to diffusion layer
wiring. With this operation, the diffusion layer wiring D02 and D04
is charged to 5 V if they are program-selected or to 0 V if they
are program-unselected. After that, the selected word line WL00 is
activated to 12 V at a timing of time t3, and then TR is brought to
an OFF-state at a timing of time t4 to interrupt the connection
between the control circuits and the bit lines, thereby bringing
the bit lines to a floating state. Then, at a timing of time t5, a
selected AG gate AG01 is charged to a programming voltage of 0.6 V,
for example, to supply a channel current to the selected memory
cell.
For example, if a memory cell M01 is the program-selected cell, the
voltages of D01, D02, the word line WL00, and the AG gate become 0
V, 5 V, 12 V, and 0.6 V, respectively; therefore, electrons are
injected into a floating gate. In this case, since D02 and the bit
line DL0 are in the floating state after being charged to 5 V,
voltages thereof are reduced as the channel current flows to the
memory cell M01, the voltages being ultimately reduced to 0 V.
Here, during a period when the voltages of D02 and the bit line DL0
are biases sufficient to generate hot electrons, the electron
injection into the floating gate is maintained. On the other hand,
if the memory cell M01 is program-unselected, the voltages of D0,
D02, the word line Wl00, and AG become 0 V, 0 V, 12 V, and about
0.6 V, respectively; therefore, the channel current does not flow
so that electrons are not injected into the floating gate. Although
the node for accumulating the charge is the diffusion layer wiring
portion in the first embodiment, the nodes are the diffusion layer
wiring portion and the bit line portion in the present embodiment;
therefore, the present invention can accumulate much more charges.
For example, stray capacitance of the bit line portion is about 1.0
pF, and a total of the stray capacitance of the bit line portion
and the diffusion layer wiring portion is about 1.3 pF. Therefore,
much more charges are accumulated in the present embodiment as
compared with the first embodiment, thereby making it possible to
inject much more electrons into the floating gate by one electron
injection operation.
Then, AG01 is inactivated at a timing of time t6, and WL00 is
inactivated at a timing of time t7. ST03 is then activated at a
timing of time t8 to connect the diffusion layer wirings D02 and
D04 to the common source line SS, so that the diffusion layer
wirings are discharged to 0V and the operation of injecting
electrons into the floating gate is completed.
After that, an operation for verifying whether or not a threshold
voltage has reached a desired value is performed, and the electron
injection operation is repeated for memory cells in which the
desired threshold voltage has not been attained. The programming is
terminated when the desired threshold voltage is attained in each
of the memory cells to which the programming must be performed.
Thus, since a programming variation due to a variation in AG bias
is reduced, thereby reducing the number of programming
verification, it is possible to shorten time required for
programming.
Further, since it is unnecessary to activate the internal power
source for supplying the programming bias of 5 V during the
programming, it is possible to suppress the variation in
programming characteristic caused by operation noise of the
internal power source and to reduce the power consumption by
maintaining the internal power source in the inactive state.
In the foregoing, the operation of increasing the threshold voltage
is described as the programming; however, it is possible to apply
the threshold value increasing operation to an erase operation.
Also, in the case of the multilevel memory, wherein a memory cell
is capable of having two or more threshold voltages, the effect of
the present method become more prominent. In the multilevel memory,
since it is necessary to control with high precision the threshold
voltages for data, the number of threshold voltage verification is
increased if the programming variation is great, thereby reducing
the programming speed. In the present method, the programming
variation is reduced to suppress the number of threshold voltage
verification, thereby increasing the programming speed.
Further, the present embodiment has an advantage that much more
charges can be accumulated as compared with the first embodiment so
that much more electrons can be injected into the floating gate by
one electron injecting operation.
In addition, the same effect is achieved if the memory cell is the
one wherein the charge accumulation node of the present embodiment
is formed of a silicon nitride film or a multilayer of a silicon
nitride film and a silicon oxide film in place of the polysilicon
film.
Alternatively, the same effect is achieved if the memory cell is
the one wherein the charge accumulation node is formed of a
plurality of polysilicon particles in the form of dots in place of
the polysilicon film.
Further, in the case of using the silicon nitride film or the
dotted polysilicon particles as the charge-injected portion, the
same effect is achieved if the memory cell is the one wherein the
polysilicon gate having the same function as that of the third gate
is provided on each side of the charge-injected portion via a
silicon oxide film. In this case, the charge-injected portion can
discretely retain charges at two places close to the respective
adjacent polysilicon gates, and thus, the multilevel memory is
realized by the different charge accumulation places.
<Embodiment 3>
A third embodiment will be described with reference to FIGS. 6 and
9 to 11. FIG. 9 is a circuit diagram which is directed to one of
memory cells of the memory array structure shown in FIG. 2, and
FIG. 10 is a diagram showing a programming operation scheme of the
present embodiment. As shown in FIG. 10, an internal power source
PROG for supplying a channel current is activated to 5 V at a
timing of time t0; STS and STD, which are switching MOS on the
source side and the drain side, respectively, of a selected memory
cell, are brought to an ON-state at a timing of t1; and a
programming voltage of 12 V is applied to a word line WL of the
selected memory cell at a timing of time t2. Then, STS which is the
switching MOS on the source side is brought to an OFF-state at a
timing of time t3 to bring a node NS to a floating state. After
that, a voltage of about 0.6 V is biased to AG of the selected
memory cell at a timing of time t4 so that a current starts to flow
to the memory cell from the internal power source PROG via the
switching MOS STD. Here, hot electrons generated in a channel
region of the memory cell are injected into a floating gate to
achieve programming. A voltage of a node ND on the drain side is
constant at 5 V, for example, while a voltage of the node NS on the
source side is increased with the flow of the channel current. When
the MOS formed of an AG gate portion is brought to an OFF-state due
to the increase in the voltage of the node NS, the programming is
stopped.
The first and second embodiments described above is characterized
in that the programming is performed by the use of hot electrons
generated by supplying the accumulated charge to the memory cell,
the present embodiment is characterized by accumulating charges at
certain capacitance via a memory cell so as to perform the
programming by the use of hot electrons generated by the charge
accumulation.
The present embodiment will be described in more details with
reference to FIGS. 6 and 11. Shown in FIG. 11 is a programming
operation of the present invention in the array structure shown in
FIG. 6. The array structure of FIG. 6 is the same as that as
described in Embodiment 1, wherein it is assumed that a memory cell
for programming is a word line WL00. After a programming command
and programming data are inputted, ST02 is activated at a timing of
time t0 to connect diffusion layer wiring D01 and D03 to a common
source line. Then, TR is brought to a High state at a timing of
time t1 to connect control circuits PC0 and PC1 to metal bit lines
DL0 and DL1, respectively. Here, voltages applied from the control
circuits are such that a programming voltage of 5 V, for example,
is applied to the bit line for a program-selected memory cell and a
voltage of 0 V is applied to the bit line for a program-unselected
memory cell. After the program-selected bit line is charged to 5 V,
a gate signal ST00 of a selected transistor is brought to an
ON-state at a timing of time t2 to connect the bit line to
diffusion layer wiring. With this operation, the diffusion layer
wiring D02 and D04 are charged to 5 V if they are program-selected
or to 0 V if they are program-unselected. After that, the selected
word line WL00 is activated to 12 V at a timing of time t3, and
then ST02 is brought to an OFF-state at a timing of time t4 to
interrupt the connection between the common source line and the
diffusion layer wiring D01 and D03. Then, a selected AG gate AG01
is charged to a programming voltage of 0.6 V, for example, at a
timing of time t5 to let a channel current flow to the selected
memory cell. For example, if a memory cell M01 is a
program-selected cell, the voltages of D01, D02, the word line
WL00, and the AG gate become 0 V, 5 V, 12 V, and 0.6 V,
respectively; therefore, electrons are injected into a floating
gate. In this case, as the channel current flows to the memory cell
M01, a voltage of D01 is increased to bring MOS formed of the AG
gate portion to an OFF-state, thereby stopping the programming. On
the other hand, if the memory cell M01 is program-unselected, the
voltages of D01, D02, the word line Wl00, and AG become 0 V, 0 V,
12 V, and about 0.6 V, respectively; therefore, the channel current
does not flow and electrons are not injected into the floating
gate. Then, AG01 is inactivated at a timing of time t6, and WL00
and TR are inactivated at a timing of time t7. ST01, ST02, and ST03
are activated at a timing time t8 to discharge the bit line and the
diffusion layer wiring to 0V, thereby completing the operation of
injecting electrons into the floating gate.
After that, an operation for verifying whether or not a threshold
voltage has reached a desired value is performed, and the electron
injection operation is repeated for memory cells in which the
desired threshold voltage has not been attained. The programming is
terminated when the desired threshold voltage is attained in each
of the memory cells to which the programming must be performed.
Thus, since a programming variation due to a variation in AG bias
is reduced, thereby reducing the number of programming
verification, it is possible to reduce time required for
programming.
In the foregoing, the operation of increasing the threshold voltage
is described as the programming; however, it is possible to apply
the threshold value increasing operation to an erase operation.
Also, in the case of the multilevel memory, wherein a memory cell
is capable of having two or more threshold voltages, the effect of
the present method become more prominent. In the multilevel memory,
since it is necessary to control the threshold voltages for data
with high precision, if the programming variation is great, the
number of threshold voltage verification is increased, thereby
reducing the programming speed. In the present method, the
programming variation is reduced to suppress the number of
threshold voltage verification, thereby increasing the programming
speed.
In addition, the same effect is achieved if the memory cell is the
one wherein the charge accumulation node of the present embodiment
is formed of a silicon nitride film or a multilayer of a silicon
nitride film and a silicon oxide film in place of the polysilicon
film.
Alternatively, the same effect is achieved if the memory cell is
the one wherein the charge accumulation node is formed of a
plurality of polysilicon particles in the form of dots in place of
the polysilicon film.
Further, in the case of using the silicon nitride film or the
dotted polysilicon particles as the charge-injected portion, the
same effect is achieved if the memory cell is the one wherein a
polysilicon gate having the same function as that of the third gate
is provided at each side of the charge-injected portion via a
silicon oxide film. In this case, the charge-injected portion can
discretely retain charges at two places close to the respective
adjacent polysilicon gates, and thus, the multilevel memory is
realized by the different charge accumulation places.
<Embodiment 4>
A fourth embodiment of the present invention will be described with
reference to FIGS. 6 and 12. FIG. 12 is a diagram showing a
programming operation scheme of the present invention in the array
structure shown in FIG. 6. The array structure of FIG. 6 is the
same as that described in Embodiment 1, and it is assumed that the
memory cell for programming is a word line WL00.
After a programming command and programming data are inputted, a
common source line SS is activated to 5 V at a timing of time t0.
Then, TR is activated at a timing of time t1 to connect control
circuits to metal bit lines. Here, the control circuits output
voltages to the bit lines for the programming data. For example, a
voltage of 0V is biased to the bit line for the program-selected
memory cell, and a voltage of 1 V is biased to the bit line for the
program-unselected memory cell. After that, the gate signal ST03 of
the selected transistor is brought to a High state at a timing of
time t2 to charge diffusion layer wirings D02 and D04 to 5 V. Then,
a gate signal ST01 of a selected transistor is brought to a High
state at a timing of time t3 to connect the bit lines DL0 and DL1
to diffusion layer wiring D01 and D03, respectively. Here, a
voltage of each of the program-selected D01 and D03 of is 0 V,
while a voltage of each of the program-unselected D01 and D03 is 1
V.
After activating the selected word line WL00 to a programming
voltage of 12 V, for example, at a timing of time t4, TR is brought
to a LOW state at a timing of time t5 to disconnect the diffusion
layer wiring D02 and D04 from the common source line. Then, a
programming voltage of about 0.6 V, for example, is biased to AG01
which is a selected AG gate. If a memory cell M01 is the
program-selected cell, voltages applied to DL0 and DL1, D02, the
word line WL00, and the AG gate are 0 V, 5 V, 12 V, and 0.6 V,
respectively; therefore, electrons are injected into the floating
gate. In this case, since the bit line DL0 is in a floating state,
charges are charged to increase a voltage as the channel current
flows to the memory cell M01 to bring MOS formed of a AG gate
portion to OFF-state, thereby stopping the programming. Although
the node for accumulating the charge is the diffusion layer wiring
portion in the third embodiment, the diffusion layer wiring portion
and the bit line portion are used as the nodes in the present
embodiment; therefore, the present invention can accumulate much
more charges. For example, stray capacitance of the bit line
portion is about 1.0 pF, and a total of the stray capacitance of
the bit line portion and the diffusion layer wiring portion is
about 1.3 pF. Therefore, much more charges are accumulated in the
present embodiment as compared with the first embodiment, thereby
making it possible to inject much more electrons into the floating
gate by one electron injection operation.
On the other hand, if the memory cell M01 is program-unselected,
voltages biased to DL1 and DL2, D02, the word line WL00, and AG01
are 1 V, 5 V, 12 V, and about 0.6 V, respectively; therefore, no
channel current flows and no electron injection to the floating
gate occurs.
AG01 is inactivated at a timing of time t7, and then WL00 and SS
are inactivated at a timing of time t8. ST00 is then activated at a
timing of time t9 to discharge the bit line and the diffusion layer
wiring to 0 V, thereby completing the operation of injecting
electrons into the floating gate.
After that, an operation for verifying whether or not a threshold
voltage has reached a desired value is performed, and the electron
injection operation is repeated for memory cells in which the
desired threshold voltage has not been attained. The programming is
terminated when the desired threshold voltage is attained in each
of the memory cells to which the programming must be performed.
Thus, since a programming variation due to a variation in AG bias
is reduced, thereby reducing the number of programming
verification, it is possible to reduce time required for
programming.
In the foregoing, the operation of increasing the threshold voltage
is described as the programming; however, it is possible to apply
the threshold value increasing operation to an erase operation.
Also, in the case of the multilevel memory, wherein a memory cell
is capable of having two or more threshold voltages, the effect of
the present method becomes more prominent. In the multilevel
memory, since it is necessary to control the threshold voltage for
the data with high precision, if the programming variation is
great, the number of threshold voltage verification is increased
thereby reducing the programming speed. In the present method, the
programming variation is reduced to suppress the number of
threshold voltage verification, thereby increasing the programming
speed.
Further, the present embodiment can charge much more charges as
compared with the third embodiment and has the advantage of
injecting much more electrons into the floating gate by one
electron injection operation.
In addition, the same effect is achieved if the memory cell is the
one wherein the charge accumulation node of the present embodiment
is formed of a silicon nitride film or a multilayer of a silicon
nitride film and a silicon oxide film in place of the polysilicon
film.
Alternatively, the same effect is achieved if the memory cell is
the one wherein the charge accumulation node is formed of a
plurality of polysilicon particles in the form of dots in place of
the polysilicon film.
Further, in the case of using the silicon nitride film or the
dotted polysilicon particles as the charge-injected portion, the
same effect is achieved if the memory cell is the one wherein a
polysilicon gate having the same function as that of the third gate
is provided on each side of the charge-injected portion via a
silicon oxide film. In this case, the charge-injected portion can
discretely retain charges at two places close to the respective
adjacent polysilicon gates, and the multilevel memory is realized
by the different charge accumulation places.
<Embodiment 5>
A fifth embodiment will be described with reference to FIGS. 13 to
15. FIG. 13 is a diagram showing a circuit structure according to
the present embodiment. A memory array MA shown in FIG. 13 is known
as an SST type memory cell which is disclosed in, for example, IEEE
ELECTRON DEVICE LETTERS, VOL. 21, NO. 7, JULY 2000, p. 359 p. 361,
wherein programmable/erasable memory cells are arranged in the form
of arrays. Programming in the above memory cell is executed by
applying voltages of 0 V, about 2 V, about 0.5 V, and about 10 V to
a well, a control gate, a drain, and a source, respectively,
thereby injecting electrons into a floating gate by a SSI (Source
Side Injection) method. An erase operation is executed by applying
a voltage of about 12 V to the control gate and a voltage of 0 V to
the source, the drain, and the well, thereby discharging electrons
which have been injected in the floating gate to the control gate.
A read operation is executed by applying voltages of about 3 V and
about 2 V to the control gate and the drain, respectively, and a
voltage of 0 V to the source and the well, and by detecting a
memory cell current.
In the programming operation of the memory cell, since the control
gate is activated by applying a voltage which is approximately
equal to the threshold voltage of a MOS transistor formed of a
control gate portion, a dimensional variation in the control gates
and a variation in the applied voltages may greatly influence a
programming characteristic of the memory cell. This is the same as
the case of the memory cell with AG gate described in the first to
fourth embodiments. The present embodiment is characterized by
providing a capacitance element between a programming control
circuit and a memory cell so that the variation in the programming
can be reduced in the memory array structure described above.
Hereinafter, a specific description of the present embodiment will
be given with reference to FIGS. 13 and 14. Denoted by M00 is a
program-selected cell. After a programming command and programming
data are inputted, source lines SS0 and SS1 are activated to about
10 V at a timing of time t0. Then, voltages for the programming
data are outputted from a programming control circuit 50 to bit
lines at a timing of time t1. Under an assumption that the bit line
DL0 is a selected bit line and the bit line DL1 is an unselected
bit line, the voltages applied to DL0 and DL1 are 0.5 V and 2 V,
respectively. After the completion of charging to the bit lines,
the connections between the programming control circuit 50 and the
bit lines are interrupted, and, after that, a selected control gate
WL0 is activated to 2 V at a timing of time t2. Since voltages of 2
V, 10 V, 0.5 V, and 0V are biased to the control gate, the source,
the drain, and the well, respectively, in the selected memory cell
M00, electrons are injected into the floating gate. On the other
hand, in a program-unselected memory cell M10, since a voltage of 2
V is applied to the drain although voltages of 2 V, 10 V, and 0 V
are applied to the control gate, the source, and the well,
respectively, no channel current flows to the memory cell so that
programming is not executed. In this case, since the capacitance
element C0 is connected to DL0, charges are accumulated in the
capacitance element C0 with the flow of a channel current to the
memory cell M00. When a voltage of DL0 is increased to a certain
level by accumulating the charge in C0, the memory cell M00 is
brought to OFF-state to stop the programming. After that, the
control gate WL0 is inactivated at a timing of time t3, and the
source lines SS0 and SS1 as well as the bit lines DL0 and DL1 are
discharged to 0 V so that the electron injection operation is
terminated.
After this, an operation for verifying whether or not a threshold
voltage has reached a desired value is performed, and the electron
injection operation is repeated for memory cells in which the
desired threshold voltage has not been attained. The programming is
terminated when the desired threshold voltage is attained in each
of the memory cells to which the programming must be performed.
Thus, since a programming variation due to a variation in control
gate bias can be reduced, thereby reducing the number of
programming verification, it is possible to reduce time required
for programming.
The above effect can be realized by a structure shown in FIG. 15
where capacitance C for programming is provided between a decoder
circuit 51 for decoding a plurality of bit lines and a programming
control circuit. In such structure, it is possible to increase a
value of the programming capacitance because the capacitance is
shared by the plurality of bit lines.
The capacitance element described above may be stray capacitance of
the bit line formed of metal. Also, the structure may be MIM
(Metal-Insulator-Metal) structure wherein an insulator film is
sandwiched between metal layers or MOS capacitance structure which
is widely used in ordinary circuit structures.
In the foregoing, the operation of increasing the threshold voltage
is described as the programming; however, it is possible to apply
the threshold value increasing operation to an erase operation.
Also, in the case of the multilevel memory, wherein a memory cell
is capable of having two or more threshold voltages, the effect of
the present method become more prominent. In the multilevel memory,
since it is necessary to control the threshold voltages for the
data with high precision, if the programming variation is great,
the number of threshold voltage verification is increased thereby
reducing the programming speed. In the present method, the
programming variation is reduced to suppress the number of
threshold voltage verification, thereby increasing the programming
speed.
In addition, the same effect is achieved if the memory cell is the
one wherein the charge accumulation node of the present embodiment
is formed of a silicon nitride film, a multilayer of a silicon
nitride film and a silicon oxide film, or a plurality of
polysilicon particles in the form of dots in place of the
polysilicon film.
Further, in the case of using the silicon nitride film or the
dotted polysilicon particles as the charge-injected portion, the
same effect is achieved if the memory cell is the one wherein a
polysilicon gate having the same function as that of the third gate
is provided on each side of the charge-injected portion via a
silicon oxide film. In this case, the charge-injected portion can
discretely retain charges at two places close to the respective
adjacent polysilicon gates, and the multilevel memory is realized
by the different charge accumulation portions.
Although the description has been made of the SST memory cell in
the foregoing, the present embodiment is applicable to a virtual
ground type memory cell using a three-layer polysilicon gate, which
is disclosed in Japanese Patent No. 2694618, and an ordinary NOR
memory cell. Note that, since the NOR memory cell typically has as
relatively small as injection efficiency of about 10.sup.-5 to
10.sup.-6, it is necessary to increase a value of the programming
capacitance. It is desirable to provide capacitance of about 100 pF
to 1 nF. Such capacitance cannot usually be realized by diffusion
layer capacitance or stray capacitance of bit line, but realized by
the capacitance of the MIM structure, MOS capacitance, or external
capacitance provided externally to a chip.
<Embodiment 6>
A sixth embodiment of the present invention will be described with
reference to FIGS. 16 to 20. FIG. 16 is a cross-sectional view
showing a memory cell of the present embodiment. The memory cell
has a well 601 formed in a substrate 600, a source diffusion layer
region 606 and a drain diffusion layer region 607 both of which are
formed in the well, a silicon oxide film 602, a silicon nitride
film 603, a silicon oxide film 604, a polysilicon gate 605 which
will be formed into a first gate, and a polysilicon gate 609 which
will be formed into a second gate; the silicon oxide film 602, the
silicon nitride film 603, and the silicon oxide film 604 being
formed in this order above the well, the polysilicon gate 605 being
formed on the silicon oxide film 604, and the polysilicon gate 609
being formed above the first gate and the well via a silicon oxide
film 608. The basic operation of the memory cell will be described
with reference to FIG. 17. FIG. 17 is a diagram showing an array
structure in which the memory cells are arranged in the same manner
as that of Embodiment 5, wherein an erase operation, programming
operation, and read operation are illustrated. As shown in FIG.
17(a), the programming operation is performed by applying voltages
of about 8 V, about 2 V, about 6 V, and about 0.5 V to the first
gate, the second gate, the source, and the drain, respectively, of
a selected memory cell to generate hot electrons and injecting the
hot electrons into the silicon nitride film serving as a
charge-injected portion to trap the electrons, thereby increasing a
threshold voltage. Referring to FIG. 17(b), the erase operation is
performed by applying voltages of about 12 V and about 6 V to the
first gate and the second gate, respectively, of the selected cell
to discharge the electrons trapped in the silicon nitride film to
the first gate, thereby lowering the threshold voltage. Referring
to FIG. 7(c), the read operation is performed by applying voltages
of about 2 V, 0 V, and about 2 V to the drain, the source and the
first gate, and the second gate, respectively, to judge data on the
basis of an amount of current supplied to the memory cell. A
current flowing through the memory cell in a programming state is
small because the electrons are trapped by the trap of the silicon
nitride film which is the charge-injected portion, while a current
flowing through the memory cell in an erasing state is larger than
that flowing through the memory cell in the programming state
because no electron is trapped in the charge-injected portion.
In the programming operation of the memory cell, since the
operation is performed by applying to a second gate portion the
voltage approximately equal to a threshold voltage of a MOS
transistor formed of the second gate portion, a dimensional
variation in the second gates and a variation in the applied
voltages greatly influence a programming characteristic of the
memory cell. This is the same as the case of the memory cell with
AG gate of the first to fourth embodiments. The present embodiment
is characterized by providing a capacitance element between a
programming control circuit and a memory cell so that the variation
in the programming is reduced in the memory array structure.
Hereinafter, a specific description of the present embodiment will
be given with reference to FIGS. 18 and 19. Denoted by M00 is a
program-selected cell. After a programming command and programming
data are inputted, source lines SS0 and SS1 are activated to about
6 V at a timing of time t0. Then, voltages for the programming data
are outputted from the programming control circuit 60 to bit lines
at a timing of time t1. Under an assumption that the bit line DL0
is a selected bit line and the bit line DL1 is an unselected bit
line, the voltages applied to DL0 and DL1 are 0.5 V and 2 V,
respectively. After the completion of charging the bit lines, the
connections between the programming control circuit 60 and the bit
lines are interrupted, and then a selected control gate WL0 is
activated to 2 V at a timing of time t2. Since voltages of 2 V, 6
V, 0.5 V, and 0V are applied to the control gate, the source, the
drain, and the well, respectively, in the unselected memory cell
M00, electrons are injected into the floating gate. On the other
hand, since a voltage of 2 V is applied to the drain although
voltages of 2 V, 6 V, and 0 V are applied to the control gate, the
source, and the well, respectively, in an unselected memory cell
M10, programming is not executed. In this case, the capacitance
element C0 is connected to DL0 so that charges are accumulated in
the capacitance C0 as a channel current flows through the memory
cell M00. When a voltage of DL0 is increased to a certain level by
the accumulation of charges in C0, the memory cell M00 is brought
to an OFF-state thereby stopping the programming. After that, the
control gate WL0 is inactivated at a timing of time t3, and the
source lines SS0 and SS1 as well as the bit lines DL0 and DL1 are
discharged to 0 V at a timing of time t4 so that the electron
injection operation is terminated.
After that, an operation for verifying whether or not a threshold
voltage has reached a desired value is performed, and the electron
injection operation is repeated for memory cells in which the
desired threshold voltage has not been attained. The programming is
terminated when the desired threshold voltage is attained in each
of the memory cells to which the programming must be performed.
Thus, since the programming variation due to the variation in
control gate bias is reduced, thereby reducing the number of
programming verification, it is possible to reduce time required
for programming.
The above effect can be realized by a structure shown in FIG. 20
where programming capacitance C is provided between a decoder
circuit 61 for decoding a plurality of bit lines and a programming
control circuit. In such structure, it is possible to increase a
value of the programming capacitance because the capacitance is
shared by the plurality of bit lines.
The capacitance element described above may be stray capacitance of
the bit line formed of metal.
In the foregoing, the operation of increasing the threshold voltage
is described as the programming; however, it is possible to apply
the threshold value increasing operation to an erase operation.
Also, in the case of the multilevel memory, wherein a memory cell
is capable of having two or more threshold voltages, the effect of
the present method become more prominent. In the multilevel memory,
since it is necessary to control the threshold voltages for data
with high precision, if the programming variation is great, the
number of threshold voltage verification is increased, thereby
reducing the programming speed. In the present method, the
programming variation is reduced to suppress the number of
threshold voltage verification, thereby increasing the programming
speed.
In addition, the same effect is achieved if the memory cell is the
one wherein the charge accumulation node of the present embodiment
is formed of a plurality of polysilicon particles in the form of
dots in place of the silicon nitride film.
Further, in the case of using the silicon nitride film or the
dotted polysilicon particles as the charge-injected portion, the
same effect is achieved if the memory cell is the one wherein a
polysilicon gate having the same function as that of the third gate
is provided at each of the sides of the charge-injected portion via
a silicon oxide film. In this case, the charge-injected portions
can discretely retain charges at two places close to the respective
adjacent polysilicon gates, and the multilevel memory is realized
by the different charge-injected places.
<Embodiment 7>
A seventh embodiment of the present invention will be described
with reference to FIGS. 21 and 22. In the array structure shown in
FIG. 6, if a memory sell unselected for reading-out has a negative
threshold voltage, conduction to the unselected memory cell is
established when a voltage of an unselected word line is 0 V,
thereby preventing a threshold voltage of a selected memory cell
from being detected precisely. Therefore, the threshold voltage of
the memory cell must be maintained at 0 V or more.
In the case of performing an erase operation by reducing a
threshold voltage of a memory cell, there is known a method of
applying a negative high voltage, e.g., -18 V, to a target for
erase, e.g., a word line so as to reduce the threshold voltage of
the memory cell by the word line unit. In this case, application of
erasing bias and threshold voltage verification operations are
repeated until the threshold voltages of all the memory cells to be
erased become VE1 or less as shown in FIG. 21. As a result, it is
possible that the threshold voltages after erasing have
distribution 1 shown in FIG. 21 and threshold voltages of a part of
the memory cells becomes lower than 0 V. As mentioned above, since
the normal reading-out is prevented if a memory cell exists which
has the threshold voltage lower than 0 V, it is necessary to
perform a post-erase operation for increasing the threshold voltage
to a level equal to 0 V or more, subsequently to the erase
operation. Here, the post-erase operation is effected by setting
the threshold voltages of all the memory cells to which the erasing
has been performed to a level equal to VE2 or more.
In the post erase operation, since the excessively increased
threshold voltage can possibly be confused with a voltage in the
programming state, a threshold voltage distribution 2 after the
post-erase should be suppressed to a voltage VE3 which is lower
than a read voltage VREAD. If a variation in post-erase
characteristic is great, a memory cell having a threshold voltage
higher than VE3 may occur incidentally and a speed of the erase
operation as a whole may be reduced because it is necessary to
perform the bias application and the threshold voltage verification
repeatedly so as to narrow the threshold value to a level in the
range of VE2 to VE3.
Accordingly, if a method of any one of the first to sixth
embodiments is carried out when the post-erase operation is
performed by increasing the threshold voltage after the erase
operation has been performed by reducing the threshold voltage, it
is possible to reduce the programming variation. It is therefore
possible to suppress the occurrence of the incidental error or the
reduction in post-erase speed. Such post-erase operation will be
described with reference to a flowchart shown in FIG. 22 based on
the second embodiment.
If an erase command is inputted, a voltage of -18 V is applied to a
selected word line to reduce a threshold voltage of a memory cell.
After that, threshold voltage verification is performed to judge if
the threshold voltages of all the memory cells to be erased are VE1
or less. In the case of NG, an erasing pulse is applied again to
reduce the threshold voltage of the relevant memory cell. In this
case, if the number of repetition exceeds a predetermined value
KMAX, a Fail flag is outputted as an erase failure to the external
to terminate the erase. When all the threshold voltages become VE1
or lower, the post-erase operation is performed. Specifically, a
bit line is connected to a power source so that the bit line is
charged to 5 V. Then, the power source and the bit line are
disconnected to bring the bit line to a floating state. After that,
the selected word line is activated to 12 V, and an auxiliary gate
is activated to 0.6 V, so that electrons are injected into a
floating gate. After the lapse of a certain period of time, the
voltages of the word line and the auxiliary gate are deactivated to
stop the post erase operation, and then the threshold voltage
verification is performed. If the threshold voltages of all the
memory cells which are targets for the post erase have not reached
VE2, a post erase bias is applied again to the memory cell whose
threshold voltage is lower than VE2. Here, if the number of
repetition exceeds a predetermined value NMAX, the Fail flag is
outputted as the erase failure to the external to terminate the
post-erase operation.
When the threshold voltages of the memory cells which are targets
for the post erase become VE2 or more, verification of whether or
not the threshold voltages are VE3 or less is performed. If a
memory cell exists which has a threshold voltage higher than VE3,
the Fail flag is outputted as the erase failure to terminate the
post erase operation. Thus, if the threshold voltages after the
post erase are from VE2 to VE3, the erase operation is terminated
normally.
In the case of outputting the Fail flag, it is desirable that the
threshold voltages of all the memory cells to be erased are set to
a level above a predetermined voltage in advance.
Although the description has been made on the basis of the second
embodiment in the foregoing, the post erase operation is applicable
to the first and third to sixth embodiments.
<Embodiment 8>
An eighth embodiment of the present invention will be described
with reference to FIGS. 23 and 24. In the case of performing
programming by increasing a threshold voltage of a memory cell
according to any one of the first to seventh embodiments, the
electron injection efficiency deteriorates with the progress of the
programming to the memory cell and the increase in threshold
voltage. Therefore, if the bias of a memory cell on which the
programming is performed earlier in the order of programming is set
to a value which prevents a threshold voltage from exceeding a
desired value, it is necessary to perform the pulse application and
the threshold voltage verification repeatedly until a memory cell
on which the programming is performed later reaches a desired
threshold voltage, thereby reducing the programming speed.
Thus, in order to maintain the electron injection efficiency at a
constant value, it is necessary to increase the bias to be applied
to a memory cell for each programming pulse. Shown in FIG. 23 is an
example of the bias increment, wherein a voltage to be applied to a
selected word line is increased as shown by VW1, VW2, VW3, etc.
with the increase in programming pulse cycles. Since the efficiency
of injecting electrons into a floating gate is increased with an
increase in a word line voltage, it is possible to perform
programming with the electron injection efficiency being maintained
constant by properly setting VW1, VW2, VW3, etc. in accordance with
a memory cell characteristic.
Further, a method shown in FIG. 24 is also effective wherein a
voltage to be applied to a drain is increased (VWD1, VWD2, VW3,
etc.) with the increase in programming pulse cycles. Since the
efficiency of injecting electrons into a floating gate is enhanced
with the increase in the drain voltage, it is possible to perform
programming with the electron injection efficiency being maintained
constant by properly setting VWD1, VWD2, VW3, etc. in accordance
with a memory cell characteristic.
<Embodiment 9>
A ninth embodiment of the present invention will be described with
reference to FIGS. 25 and 26. Embodiments 1 to 8 heretofore
describe the method of performing programming by supplying an
injected charge to a memory cell and the method of performing
programming by charging certain capacitance via a memory cell. In
both of the methods, the verification operation of a threshold
voltage is performed every time an electron injection operation is
performed as shown in FIG. 25. In these methods, if a change in the
threshold voltage in one electron injection operation is
inadequate, the number of repetition is increased to sometimes
cause a reduction in programming speed. The present embodiment is
characterized by preventing such reduction in programming speed by
repeating the threshold voltage verification after performing the
electron injection operation at least once. Shown in FIG. 26 is the
programming method of the present embodiment.
The present embodiment will be described with reference to FIG. 26.
After repeating the electron injection operation N=f(k) times
(where k is the number of threshold voltage verification, f(k) is a
function of k), the threshold voltage verification operation is
repeated until programming to memory cells which are the target for
programming is completed or the number of threshold voltage
verification reaches a predetermined value of Kmax, thereby
finishing the programming. N is a function of k, and it is possible
to set a value of N to an arbitrary number in accordance with a
memory cell characteristic. For example, in view of the fact that
the electron injection into floating gate becomes difficult with
the increase in the threshold voltage, the threshold voltage
increment may be maintained as constant as possible by increasing
the number of electron injection operation with the increase in the
number of threshold voltage verification.
<Embodiment 10>
A ninth embodiment of the present invention will be described with
reference to FIGS. 27 to 31. The foregoing descriptions of
Embodiments 1 to 9 have been directed to the case where one memory
cell has one bit of information, whereas the case of a multilevel
memory wherein one memory cell has three or more threshold voltage
states will be described in the present embodiment. FIG. 27 is a
diagram showing a relationship between a state of threshold voltage
and data in a multilevel memory of 2 bit/cell. It is possible to
store 2 bits of 01, 00, 10, or 11 by setting the threshold voltage
of a memory cell to a first state, a second state, a third state,
or a fourth state, thereby realizing the reduction in bit cost. In
FIG. 27, the fourth state corresponds to an erase state.
Referring to FIGS. 27 and 28, an example of ordinary programming
method in a 2 bit/self flash memory will be described. The flash
memory shown in FIG. 28 can take the threshold voltage states shown
in FIG. 27. When a programming command is inputted externally,
programming data are taken into a data buffer of a chip. Then, a
bit line associated with a memory cell on which the programming is
performed at the first state is connected to a power source to be
activated to about 5 V. After that, a selected word line and a
selected AG are activated to about 12 V and about 0.6 V,
respectively, so that part of hot electrons generated at a channel
portion of the memory cell is injected into a floating gate,
thereby increasing the threshold voltage of the memory cell. After
the selected word line and the selected AG are inactivated and the
bit line is discharged, read from the memory cell is performed.
Then it is verified if the programming at the first state has been
completed or not. The verification operation is performed by
applying Vpref1 to the selected word line and judging whether the
memory cell is brought to an ON-state. The programming operation
and the verification operation are repeated until all the memory
cells on which the programming is performed at the first state
reach the predetermined threshold voltage.
After the completion of the programming at the first state,
programming at the second state is started. A bit line associated
with the memory cell on which the programming is performed at the
second state is connected to the power source to be activated to
about 5 V. Then, a selected word line and a selected AG are
activated to about 12 V and about 0.6 V, respectively, so that part
of hot electrons generated in a channel portion of the memory cell
is injected into a floating gate, thereby increasing the threshold
voltage of the memory cell. After the selected word line and the
selected AG are inactivated and the bit line is discharged, read
from the memory cell is performed and then it is verified whether
or not the programming at the second state has been completed. The
verification operation is performed by applying Vpref2 to the
selected word line and judging whether the memory cell is brought
to an ON-state. The programming operation and the verification
operation are repeated until the memory cells on which the
programming is performed at the second state reach the
predetermined threshold voltage.
After the completion of the programming at the second state,
programming at the third state is started. A bit line associated
with the memory cell on which the programming is performed at the
second state is connected to the power source to be activated to
about 5 V. Then, a selected word line and a selected AG are
activated to about 12 V and about 0.6 V, respectively, so that part
of hot electrons generated in a channel portion of the memory cell
is injected into a floating gate, thereby increasing a threshold
voltage of the memory cell. After the selected word line and the
selected AG are activated and the bit line is discharged, read from
the memory cell is performed. Then it is verified whether or not
the programming at the third state has been completed. The
verification operation is performed by applying Vpref3 to the
selected word line and judging whether or not the memory cell is
brought to an ON-state. The programming operation and the
verification operation are repeated until all the memory cells on
which the programming is performed at the third state reach the
predetermined threshold voltage.
As described above, the programming in the multilevel memory is
executed by repeating the electron injection into the floating gate
and the verification operation for each threshold voltage state.
However, as described above, if there is the variation in the
programming characteristic of the memory cell, the number of
repetition (hereinafter referred to as "the number of
verification") is increased. For example, if there is a variation
of the order of 1.3 in the memory cell programming characteristic,
about 12 times of verifications are required for each of the
states, so that the number of verification for three states reaches
36 times. Thus, particularly in the multilevel memory, if there is
the variation in the programming characteristic of the memory cell,
the problem of the increase in the programming period with the
increase in the number of verification becomes more prominent.
Shown in FIG. 29 is a flowchart of the case where the programming
method of the second embodiment is applied to the 2 bit/cell. When
a programming command is inputted externally, programming data are
taken into a data buffer of a chip. Then, a bit line associated
with a memory cell on which the programming is performed at the
first state are connected to a power source to be activated to
about 5 V, and a selected word line is activated to 12 V and then
disconnected from the power source to come in a floating state.
After that, a selected AG is activated to about 0.6 V so that a
charge which has been accumulated in stray capacitance of the bit
line is discharged via the memory cell and part of hot electrons
generated by the discharge is injected into a floating gate,
thereby increasing the threshold voltage of the memory cell. After
the selected AG and the selected word line are inactivated and the
bit line is discharged, read from the memory cell is performed.
Then it is verified whether or not the programming at the first
state has been completed. The verification operation is performed
by applying Vpref1 to the selected word line and judging whether or
not the memory cell is brought to an ON-state. The programming
operation and the verification operation are repeated until all the
memory cells on which the programming is performed at the first
state reach the predetermined threshold voltage.
After the completion of the programming at the first state,
programming at the second state is started. A bit line associated
with a memory cell on which the programming is performed at the
second state are connected to a power source to be activated to
about 5 V, and a selected word line is activated to 12 V and then
disconnected from the power source to come in a floating state.
After that, a selected AG is activated to about 0.6 V so that a
charge which has been accumulated in stray capacitance of the bit
line is discharged via the memory cell and part of hot electrons
generated by the discharge is injected into a floating gate,
thereby increasing the threshold voltage of the memory cell. After
the selected AG and the selected word line are inactivated and the
bit line is discharged, read from the memory cell is performed.
Then it is verified whether or not the programming at the second
state has been completed. The verification operation is performed
by applying Vpref2 to the selected word line and judging whether or
not the memory cell is brought to an ON-state. The programming
operation and the verification operation are repeated until the
memory cells on which the programming is performed at the second
state reach the predetermined threshold voltage.
After the completion of the programming at the second state,
programming at the third state is started. A bit line associated
with a memory cell on which the programming is performed at the
third state are connected to a power source to be activated to
about 5 V, and a selected word line is activated to 12 V and then
disconnected from the power source to be in a floating state. After
that, a selected AG is activated to about 0.6 V so that a charge
which has been accumulated in stray capacitance of the bit line is
discharged via the memory cell and part of hot electrons generated
by the discharge is injected into a floating gate, thereby
increasing the threshold voltage of the memory cell. After the
selected AG and the selected word line are inactivated and the bit
line is discharged, read from the memory cell is performed. Then it
is verified whether or not the programming at the third state has
been completed. The verification operation is performed by applying
Vpref3 to the selected word line and judging whether or not the
memory cell is brought to an ON-state. The programming operation
and the verification operation are repeated until the memory cells
on which the programming is performed at the third state reach the
predetermined threshold voltage.
As described above, it is possible to suppress the programming
characteristic variation to the order of about 0.3 as a result of
performing the programming by supplying the charge which has been
accumulated in the stray capacitance of the bit line, i.e. the
charge accumulated in the certain capacitance, to the memory cell.
In this case, the number of verification is reduced to 3 times for
each state, and the number of verification for the three states is
less than 10 times. Thus, the problem associated with the
multilevel flash memory, i.e., the reduction in the programming
speed, can be suppressed.
In the foregoing description, the voltages applied to the memory
cell at the first to third states are the same; however, the
voltages may be set as shown in FIG. 30 wherein the selected word
line voltage Vw1 at the first state programming is higher than the
selected word line voltage Vw2 at the second state programming and
VW2 is higher than the selected word line voltage Vw3 at the third
state programming. In this case, more efficient programming is
achieved since a higher voltage is applied to the memory cell in
which the threshold voltage must be changed greatly.
Further, the voltages may be set as shown in FIG. 31 wherein the
selected word line voltage Vd1 at the first state programming is
higher than the selected word line voltage Vd2 at the second state
programming and Vd2 is higher than the selected word line voltage
Vd3 at the third state programming. In this case, more efficient
programming is achieved since a higher voltage is applied to the
memory cell in which the threshold voltage must be changed
greatly.
Although the case of applying the programming method of the second
embodiment to the 2-bit/cell nonvolatile memory has been described
in the foregoing, it is possible to apply the programming method to
a multilevel memory having 3 bits or more per cell, as well.
Further, it is needless to say that the methods described in the
first and third to sixth embodiments can also be applied to a
multilevel memory having 2 bits or more per cell.
<Embodiment 11>
Shown in FIG. 32 is a multilevel programming method different from
that of the ninth embodiment. In general, in the array structure
shown in FIG. 6, the upper limit of the threshold voltage of a
memory cell is determined only by reliability. Accordingly, a
distribution width of the first state shown in FIG. 27 can greatly
be set to higher threshold voltages. Therefore, in the programming
at the first state, it is unnecessary to control the threshold
voltage highly precisely, and it is possible to complete the
programming by the application of one pulse irrespective of the
characteristic variation by increasing the programming pulse width
or the voltage.
In the first state programming, a bit line associated with a memory
cell on which the programming is performed at the first state is
connected to a power source to be activated to about 5 V and, after
that, a selected word line and a selected AG are activated to about
12 V and about 0.6 V, respectively, so that part of hot electrons
generated at a channel portion of the memory cell is injected into
a floating gate. Here, if a pulse width of the high voltage which
is applied to the selected word line is set to 10 .mu.s or more or
the selected word line voltage is set to a sufficiently high level
such as about 15 V, it is possible to complete the programming at
the first state by one programming pulse.
After the completion of the first state programming, second state
programming is started. The flow subsequent to the second state
programming is the same as that of the second embodiment, that is,
only the charge accumulated in the stray capacitance of the bit
line is supplied to the memory cell to execute the programming. A
bit line associated with a memory cell on which the programming is
performed at the second state is connected to a power source to be
activated to about 5 V, and a selected word line is activated to
about 12 V and then disconnected from the power source to come in a
floating state. After that, a selected AG is activated to about 0.6
V so that a charge which has been accumulated in stray capacitance
of the bit line is discharged via the memory cell and part of hot
electrons generated by the discharge is injected into a floating
gate, thereby increasing the threshold voltage of the memory cell.
After the selected AG and the selected word line are inactivated
and the bit line is discharged, read from the memory cell is
performed. Then it is verified whether or not the programming at
the second state has been completed. The verification operation is
performed by applying Vpref2 to the selected word line and judging
whether or not the memory cell is brought to an ON-state. The
programming operation and the verification operation are repeated
until the memory cells on which the programming is performed at the
second state reach the predetermined threshold voltage.
After the completion of the second state programming, a third state
programming is started. A bit line associated with a memory cell on
which the programming is performed at the third state is connected
to a power source to be activated to about 5 V, and a selected word
line is activated to about 12 V and then disconnected from the
power source to come in a floating state. After that, a selected AG
is activated to about 0.6 V so that a charge which has been
accumulated in stray capacitance of the bit line is discharged via
the memory cell and part of hot electrons generated by the
discharge is injected into a floating gate, thereby increasing the
threshold voltage of the memory cell. After the selected AG and the
selected word line are inactivated and the bit line is discharged,
read from the memory cell is performed. Then it is verified whether
or not the programming at the third state has been completed. The
verification operation is performed by applying Vpref3 to the
selected word line and judging whether or not the memory cell is
brought to an ON-state. The programming operation and the
verification operation are repeated until all the memory cells on
which the programming is performed at the third state reach the
predetermined threshold voltage.
It is possible to reduce the number of verification to about 7
times by establishing a state equivalent to the maximum threshold
voltage distribution by one programming pulse as described in the
present embodiment.
Although the case of applying the programming method of the second
embodiment to the 2-bit/cell nonvolatile memory has been described
in the foregoing, it is possible to apply the programming method to
a multilevel memory having 3 bits or more per cell, as well.
Further, it is needless to say that the methods described in the
first and third to sixth embodiments can also be applied to a
multilevel memory of 2 bits or more per cell.
<Embodiment 12>
Shown in FIG. 33 is a multilevel programming method different from
that of Embodiments 10 and 11. Referring to FIG. 33, reference
numeral 70 denotes a read and program control circuit, in which, in
the case of programming at the first state, a MOS transistor MVD1
is brought to an ON-state by activating SVD1 to connect a first
power source VD1 to a bit line BL. Likewise, SVD2 is activated to
connect a second power source VD2 to the bit line BL in the case of
programming at the second state, and SVD3 is activated to connect a
third power source VD3 to the bit line BL in the case of
programming at the third state. Here, the first power source has a
voltage higher than that of the second power source, and the second
power source has a voltage higher than that of the third power
source. For example, the voltages of the first, second, and third
power sources are 6 V, 5 V, and 4 V, respectively.
Shown in FIG. 34 is a programming flow in the circuit structure of
FIG. 33. When a programming command and programming data are
inputted, the bit line is connected to the power source associated
with the programming data so as to be charged to a desired voltage.
For example, the bit line of a memory cell which is selected for
the programming at the first state is charged to 6 V; the bit line
of a memory cell which is selected for the programming at the
second state is charged to 5 V; and the bit line which is selected
for the programming at the third state is charged to 4 V. After
that, the connections between the bit lines and the power sources
are interrupted to bring the bit lines to a floating state. A
selected word line is then activated to about 12 V, which is a
programming voltage, and a selected AG is activated to about 0.6 V,
so that charges which have been charged in the bit lines is
discharged via the memory cells and hot electrons generated by the
discharge are injected into floating gates. Here, the memory cells
which must be set to the higher threshold voltage have a larger
quantity of bit line-accumulated charges and a larger quantity of
injection charges into the floating gates. After the selected AG
and the selected word line are inactivated and the bit lines is
discharged, read from the memory cell is performed. Then it is
verified whether or not the programming at the first to third
states have been completed. The programming operation and the
verification operation are repeated until the memory cells on which
the programmings at the first to third states are performed reach
the predetermined threshold voltages. It is thus possible to
complete the programming at the three states substantially
simultaneously by properly setting the first to third power source
voltages.
In the present embodiment, since the programming and the
verification at the three states are performed simultaneously, the
number of required verification is about 3 times.
<Embodiment 13>
Shown in FIG. 35 is a computer system wherein the flash nonvolatile
memory according to one of Embodiments 1 to 12 is incorporated, the
system having a host CPU mutually connected to the system via a
system bus, an input/output device, a RAM, and a memory card.
The memory card includes the flash nonvolatile memory having a
large memory capacity of a several tens of gigabytes, which is used
for substituting for a hard disk storage device, for example. Since
the memory card has the benefit of the high programming speed which
is the advantage of the flash nonvolatile memory according to the
embodiments of the present invention, it has a sufficient
industrial advantage as a memory device which is an end
product.
The memory card of the present invention is not limited to a
relatively thin memory card. It is needless to say that a memory
card which is relatively larger in thickness is applicable to any
nonvolatile memory devices having an interface for a host bus
system and an intelligent controller capable of controlling the
flash nonvolatile memory by analyzing a command from the host
system.
Data to be stored for a long period of time are stored in the
nonvolatile memory device, while data to be altered frequently as
being processed by the host CPU are stored in the RAM of the
nonvolatile memory.
The card has a system bus interface to be connected to the system
bus to realize a standard bus interface such as an ATA system bus.
A controller connected to the system bus interface receives
commands and data from a host connected to the system bus as well
as host systems of the CPU and the input/output device.
If the command is a read command, the controller accesses one or
more of flash EEPROMs to transfer read data to the host system.
If the command is a write command, the controller accesses one or
more of the flash EEPROMs to store internally the write data sent
from the host system. The storage operation includes programming
operations and verification operations for relevant blocks,
sectors, and memory cells of the flash memory.
If the command is an erase command, the controller accesses to one
or more of the flash EEPROMs to erase data stored in the accessed
flash EEPROM. The erase operation includes erase operations and
verification operations for relevant blocks, sectors, and memory
cells of the flash memory.
It is needless to say that the flash nonvolatile memory according
to the embodiments of the present invention is applicable not only
to a technology for setting two threshold voltages in one memory
cell in order to store 1 bit of digital data in one memory cell but
also to a technology for setting four or more threshold voltages in
one memory cell in order to store a plurality of bits of digital
data in one memory cell.
While the foregoing embodiments are used for describing the
invention produced by the present inventors, it is apparent that
the invention is not limited to the embodiments and the invention
can be modified as long as the modification does not depart from
the spirit and scope of the invention.
For example, the present invention is applicable to a one-chip
microcomputer (semiconductor device) which is provided with a
memory cell array unit having a nonvolatile semiconductor memory
element.
According to the present invention, the programming speed or the
erasing speed of the nonvolatile semiconductor memory device is
improved and a power consumption of the nonvolatile semiconductor
memory device is reduced.
* * * * *